The first portion of the background is most closely related to a single element preturbine oxidation catalyst element for EMD turbocharged engines using twisted exhaust runners. Oxidation catalysts (OC) are used to reduce the emissions of unburned hydrocarbons (HC), carbon monoxide (CO) and certain types of particulate matter (PM). Of the aftertreatment systems used on lean burn engines, this is the simplest system as it is completely passive and practically maintenance free. In the art these are also commonly referred to as diesel oxidizing catalysts (DOC). When dealing with both diesel engines and gas engines the shortened term oxidation catalyst (OC) is the more appropriate term.
A very common use of an OC has been in the aftertreatment of diesel truck engines where the OC is placed downstream of the turbocharger, and upstream of the diesel particulate filter (DPF). Because locomotives have become so tightly packaged, there is minimal room for a downstream OC in the locomotive application. A solution to this called a V-Cat has been patented and developed by Miratech. With a V-Cat system the OC is built into the exhaust manifolds on the engine upstream of the turbocharger, hence a pre turbine OC. For packaging reasons, this system had a single OC substrate for each cylinder. ASME Paper JRCICE2007-40060 titled ‘Exhaust Emissions From a 2,850 kW EMD SD60M Locomotive Equipped With a Diesel Oxidation Catalyst’ describes the application and testing of prototype V-Cat system on a 16 cylinder EMD engine in an SD60M locomotive.
The primary parameter that determines the emissions reduction efficiency of an OC is its temperature. Results of the V-Cat testing in the ASME paper indicate the CO reduction efficiency reaches 90% at around 200 C, the peak HC reduction efficiency is approximately 50% at 320 C.
Not only does the preturbine placement of this OC system on the turbocharged EMD engine offer a solution to the packaging problem, preturbine placement of the OC has several other benefits.
As the efficiency of the OC is affected by temperature, its pre turbine placement will substantially increase its overall operating temperature. Notch 4 has a temperature drop across the turbo of 41 C. Notch 4 is when the preturbine temperature finally reaches 310 C where the OC starts to reduce HC at 50% efficiency. If the OC were downstream of the turbo in this case, it would be operating at the mid 40% range. At notch 8, the temperature difference across the turbo is 137 C.
These increases in preturbine temperature will be even more important in the future when methane becomes a regulated emission. Typical OC systems do not efficiently remove methane until 400 C. With a preturbine OC system this would start at notch 4, with a downstream OC this may not start until notch 7. When hydrocarbon emissions are regulated for natural gas engines, they regulate only non-methane hydrocarbons. In natural gas engines the methane component of total HC is typically close to 90%. Although methane is not a criteria pollutant with direct human health risks, air agencies are paying more attention to methane emissions as a potent greenhouse gas with regulations for it pending in the near future.
One interesting finding in the ASME report is that the preturbine OC actually increased the turbo inlet temperature and overall engine efficiency because of the energy released when it oxidized the CO, HC and PM matter. This led to an actual increase of engine thermal efficiency at some points even though the additional back pressure of the OC would typically cause a decrease in efficiency due to reduced airflow.
Another advantage of a preturbine OC over a downstream OC is the effect of the OC system back pressure. Downstream of the turbo, whatever back pressure the OC causes would be multiplied by the pressure ratio of the turbo turbine. So if the OC caused a pressure drop of 1.1 kPa and the turbo had a pressure ratio of 2.7, the back pressure increase in the exhaust manifold would be 2.9 kPa. In the case of the preturbine DOC, the OC pressure drop is not multiplied.
As noted, the V-Cat system tested in the ASME paper had a single OC substrate per cylinder. This system is now in production and sold exclusively by EMD the manufacturer of the EMD engines. Because of the way the exhaust manifold segments are tightly packaged across the top of the engine and there is only a short 4 inch length of common exhaust plenum before the turbo charger inlet plumbing, there was no easy way to package a single large substrate that all of the exhaust would flow through equally. The solution was to have a single OC substrate for each cylinder and therefore each cylinder would experience the same pressure drop and the engine would run smoothly. If a single large substrate was attempted and each cylinder was affected differently, performance would suffer as some cylinders would get more intake air than others.
In the report the pressure drop was measured across one substrate with the engine running, the measured pressure drop was 1.1 kPa. While accurately measured, this pressure drop is not representative of the instantaneous pressure drop that affects the scavenging of the cylinder and how much intake air is brought into the cylinder. Because the exhaust valves are open substantially and flowing exhaust for less than a ⅓ of the crank rotation, it is likely that this measured average pressure is actually ⅓ of what would be allowed with a single substrate for all of the cylinders when the exhaust pulses are all combined together into one average exhaust manifold mass flow.
Early versions of this single substrate per cylinder OC system suffered substrate failures that were attributed to the pulsing effect of the exhaust gases flowing rapidly through the substrate for only ⅓ of the crank rotation. This resulted in the substrates breaking up into small pieces and flowing through the exhaust manifold towards the turbo inlet. Fortunately the EMD engine has a built in debris screen installed in front of the turbo inlet to prevent material such as this from damaging the turbine blades. Later designs of the preturbine OC system overcame this problem by adding additional material and substrate supports to enhance the durability of the substrates.
While the existing preturbine OC solution for the EMD engine solves the packaging problem, it would be preferable if a more economical and simpler single substrate solution could be found that did not have to replace every one of the existing exhaust manifolds.
The second portion of the background is most closely related to adjustable inlet guide vanes for improved emissions in EMD locomotives. Two aftertreatment systems have been developed and tested for emissions reductions in EMD powered locomotives, and both test programs noted a spike in particulate matter (PM) emissions for notch 6 engine loading. Miratech has developed and patented a preturbine diesel oxidizing catalyst (DOC) system call the V-Cat, testing results were published in ASME Paper JRCICE2007-40060 titled ‘Exhaust Emissions From a 2,850 kW EMD SD60M Locomotive Equipped With a Diesel Oxidation Catalyst’. This system was focused on reducing PM emission and from Notch 3 to Notch 8, the system efficiency averaged over 55% except for Notch 6 where the reduction plummeted to approximately ½ that value at 27%. Overall this system reduced PM by 52%.
Engine, Fuel and Emissions Engineering has trademarked its Compact SCR and the final report documenting its system on a Metrolink passenger locomotive is available on their website at www.efee.com. Unlike the preturbine V-Cat system, the Compact SCR system was located downstream of the engine turbocharger exhaust outlet and its primary function was to reduce oxides of nitrogen (NOx). It has a secondary function of reducing PM and was capable of reducing PM by 61% on the locomotive duty cycle. The testing with the Compact SCR resulted in a similar PM emissions spike at Notch 6 as seen in the V-Cat DOC testing. Further the NOx reduction efficiency of the Compact SCR system at throttle setting of idle through Notch 2 were very low.
The notch 6 increase in PM emissions and the low load reduction in NOx reduction efficiency are due to two different characteristics of the EMD 2 stroke locomotive engine. The notch 6 PM increase is due the engine air fuel ratio starting to be less lean than optimum which decreases combustion efficiency of the diesel spray and increases soot which is a major part of diesel PM. On the other hand the low load reduction in SCR efficiency is because the engine air fuel ratio is becoming too lean and the exhaust temperature is very low.
These varied air fuel ratios are a function of the design of the turbocharged EMD 2 stroke engine. The EMD system has a unique combination supercharger and turbocharger. It is driven by the engine geartrain through a one way clutch up until the point that there is enough exhaust energy to drive the turbocharger faster than the gear train. The point where the turbo spools up is typically notch 7 and that is where the boost builds up and the engine runs a leaner air fuel ratio that produces less PM. At very low loads the engine is also at low RPM, but at these lower speeds the intake ports are open for a longer time giving the reduced boost pressure more time to drive fresh air into the cylinder. Also at these lower loads the engine actually needs less air because it is making less power and consuming less fuel. This causes the engine to take in even more air than is needed and this excess air in the combustion chamber lowers the exhaust temperature. At idle this problem is at its worst as the low RPM allows a long time for scavenging and the minimal engine fuel consumption further drives down the exhaust temperature. From a peak exhaust temperature over 500 C at notch 8, the exhaust temperature is just over 110 C at idle and 160 C at Notch 1.
Energy Conversion Inc. (ECI) in Tacoma, Wash. has had to overcome an additional problem in its conversion of these EMD 2 stroke engines to natural gas. In order to prevent detonation at high loads with natural gas, it was required to lower the compression ratio of the engine. Lowering the compression ratio at idle exacerbated the low RPM combustion temperature issue and in order to get lower emissions at idle, ECI incorporated a bank idling system where it only injected fuel into the cylinders on one side of the engine. This allowed each cylinder to operate with twice the amount of fuel and generate twice the amount of power. Every two minutes the engine would swap banks and run on the other half of the engine.
In addition to the bank idling technique, ECI devised an inlet throttle system to restrict the amount of air that the engine took in at idle to further increase the combustion temperature and increase the exhaust gas temperatures. This system had a set of rotating vanes pointing inward from a ring. This ring would be in front of the turbocharger compressor and had an open and closed setting. In the open setting the vanes would turn so that they were lined up in the direction of airflow and offered minimal resistance to the airflow. In the closed position an air actuator would rotate the vanes almost 90 degrees until the vanes touched and closed off the air passage except for the small round opening left over at the tips of the vanes.
This inlet restriction system developed by ECI is similar to variable angle inlet guide vanes used on some gas turbine engines and large stationary compressor equipment. When the guide vanes are in the neutral position they have no effect on the compressor upstream of them. When the guide vane are rotated from the neutral position they will add swirl to the flow, this swirl will have a different effect on airflow through the compressor depending on whether the swirl is turning the airflow with or against the rotation direction of the compressor impeller. If the flow is swirling in the direction of the centrifugal impeller rotation then the amount of pressure rise across the impellor will decrease as the impellor will not be able to put as much work or energy into the flow. This would tend to decrease the amount of mass flow across the compressor and the boost pressure leaving it. If the inlet guide vanes were turned in the opposite direction, the resulting airflow swirl would turn the air against the impeller rotation. This would increase the amount of work or energy that the impellor will impart into the airflow increasing the pressure rise. If the centrifugal impellor was part of a turbocharger, this increase in pressure rise would result in slowing down the impeller and turbine. This could be a form of limited waste gating for limiting or reducing the turbocharger shaft speed.
In addition to the lower compression ratio causing lower combustion temperatures at lower loads, the ECI natural gas conversion systems changed the airflow configuration of the engine enough that the stock EMD turbocharger could overspeed at notch 8. In order to control overspeed ECI added a waste gate system to bypass some of the high temp exhaust gasses and reduce turbine speeds.
Another system implemented by ECI in its conversion system is improved aftercooling of the intake air to reduce detonation at high loads. At low loads this improved intake air cooling would exacerbate the low combustion temperature issues at lower throttle settings. The solution was to revert the aftercooling system back to the original system for notches 3 down to idle where heated engine coolant is used to warm up the intake air. This required adding an actuated coolant control valve and some plumbing to control whether heated or cooled water was flowing to the liquid cooled aftercoolers.
What would be beneficial in these applications would be an airflow control system that reduced the excessively lean low load mixtures, increased boost and airflow at notch 6, and limited turbine speed in dual fuel engines at notch 8.
The third portion of the background is most closely related to a sonic and dual stage gas inlet valve. In the case of the ECI conversions systems for 2 stroke locomotive engines, a system called low pressure direct injection (LPDI) is used where the natural gas is injected directly into the cylinder during the compression stroke. What this leads to is a mixing challenge where the air and fuel have limited time to mix as the piston rises up to top dead center right before ignition.
This mixing challenge is why SwRI on their single cylinder development EMD 710 engine decided to do premixing of the air and fuel even though it would not be practical on an ‘in service’ engine as too much unburned fuel would blow through the cylinder into the exhaust while scavenging.
The in cylinder mixing issue can make prechamber operation difficult if a rich pocket of air and gas gets pushed into the prechamber which already has excess fuel in it. In this instance the prechamber will misfire and there will be no combustion for that stroke. For this reason ECI installed ‘jet caps’ on the first iteration spark ignited prechamber (SIP) system on the Napa Valley Wine train. The jet cap is an additional cap fixed over the end of the main Gas inlet valve (GIV). The GIV had a poppet valve at the end that controlled the flow of fuel gas into the combustion chamber. With the ‘jet cap’ in place, after the gas flowed thru the GIV body and past the poppet valve, it then had to flow through a small orifice at the end of the ‘jet cap’. This addressed several issues, all the gas was converged into one flow stream that now had higher velocity and was pointed away from the prechamber.
Another difference between the ECI kit and the system tested at SwRI is that the ECI system has to operate at very high Lambdas. Lambda is the ratio of the actual air/fuel ratio divided by the stoichiometric air/fuel ratio. Typical 4 stroke diesel engines operate at Lambdas around 1.9 at low load to 1.4 at full power. The SwRI single cylinder development engine didn't have to operate below 50% power. At low loads, an EMD 2 stroke locomotive operates at Lambda's above 3 and at idle the Lambda can exceed 4. At these very high lambdas it would require a larger prechamber that will produce fewer NOx emissions and have a lower thermal efficiency.
A solution to the very high Lambda value is to restrict inlet flow with a throttling system at low loads. This will allow operating the engine all the way from idle to full load with smaller volume prechambers that put out less NOx emissions and operate at higher thermal efficiency.
In a uniflow 2 stroke engine, scavenging is a process of blowing inlet air over the top of the piston at bottom dead center. This entering intake air pushes the spent combustion gasses out through the open exhaust ports at the top of the cylinder. The amount of in cylinder air motion and mixing as the piston rises in the compression stroke is proportional to how much velocity the inlet air carried in with it due to excess intake air box pressure. When the inlet is throttled to help reduce the low load air fuel Lambda, a large portion of this mixing energy is lost.
It is possible to reduce the inlet air box pressure to a low enough value that not enough inlet air enters to thoroughly scavenge the cylinder and some amount of exhaust gas will remain in the cylinder when the exhaust valves close. This effect can be desirable or have negative effects. This left over combustion gas is much hotter and less dense than the incoming air, so the resulting in cylinder air mass will now be lower and the average in cylinder temperature will be hotter at the beginning of the compression stroke. This has the double effect of both lowering the Lambda for easier combustion with less ignition energy using a smaller prechamber, and also faster and more efficient combustion because the compressed air fuel mixture is already much hotter at ignition.
This is referred to as internal exhaust gas recirculation (EGR) where exhaust gas is purposely left behind to achieve these effects. In a uniflow 2 stroke, the downside of this is much less air velocity at intake port closing. This lowered in cylinder velocity and mixing energy reduces the amount of air and fuel mixing when the natural gas is injected at low loads.
A supersonic injector for gaseous fuel engines as described in U.S. Pat. No. 6,708,905 would be a solution that offers improved mixing and a bonus of lower temperature gas when injected. This particular device has two drawbacks. First it has many machined parts with complicated features that will be costly. Second, the design has a built in cavity where residual natural gas will be compressed into and remain unburned during the combustion event. Most of the compressed gas in this cavity will become methane exhaust emissions. This release of unburned methane is both a pollution emissions problem and an energy efficiency problem.
What is desired is an economical and practical way to achieve the benefits of a high velocity and focused sonic injection nozzle without the added cavity for residual unburned methane, better mixing in the combustion chamber of a natural gas engine with direct gas injection which would allow operating a uniflow 2 cycle engine to be throttled past the point that internal EGR effects are improving combustion.
The fourth portion of this background is most closely related to prechamber cooling sleeves. Prechamber ignition systems are used to ignite air fuel mixtures that are too lean for a spark plug to ignite. The type of prechamber discussed here is a small prechamber at less than 5% of the combustion chamber clearance volume. Combustion inside of the prechamber will be easier to start and burn much more rapidly because the air fuel mixture is hotter and typically richer than the air fuel mixture in the main combustion chamber.
The cooling of a prechamber is one of the challenges, and the most challenging part of the prechamber to cool is the nozzle or tip area. This is because there is combustion happening on both sides of it. With insufficient cooling it has been documented that overheating prechambers will often melt the tip of the prechamber. Sometime before the tip actually melts, it will cause preignition which will limit how much power the engine can produce or cause the engine to run improperly during some conditions.
An improved prechamber would be a design that has better cooling for the prechamber body, prechamber tip and nozzle area while also being more economical to make.
The fifth portion of this background is most closely related prechamber cylinder deactivation on spark ignited prechamber EMD engines. Both the roots blown and turbocharged EMD engines would be good candidates for cylinder deactivation. Currently ECI used skip firing in their Spark Ignited Prechamber systems to improve combustion at very low loads where the engine operates very lean. In skip fire, the engine controller will skip actuating the main injector for a certain cylinder. This will cause the other cylinders to have to operate at a higher power to make up the lost power from the deactivated cylinders. When operating at higher power the other cylinders will need more fuel to generate it and this increase in fuel to those cylinders is what decreases how lean those cylinders are before ignition which generates higher heat release rates making the combustion events more consistent, and efficient. The control system has a strategy to alternate the deactivated cylinders to prevent any one cylinder from becoming significantly cooler than the others and also to prevent lube oil build up in that cylinder.
To keep the system simple, only the main gas injector is turned off for the cylinders that are skipped. All of the engine prechambers are still fed natural gas and the spark plugs are still fired. In the case where the prechamber supply pressure can be held constant over the entire engine operating range, the prechamber fuel supply system consists of only a single mechanical pressure regulator with a fixed setting.
Because the prechamber is still fed fuel, but the main chamber is not, there is no guarantee that the mixture in the prechamber is being burned when a cylinder is deactivated, even when the spark plug is still being fired. A portion of the fuel burned in the prechamber during normal combustion was not injected by the prechamber fuel system, but was brought in from the main chamber. When the cylinder is deactivated the air pushed into the prechamber by the piston will not have any fuel so the overall mixture in the prechamber may be too lean for the spark to burn. This is greatly dependent on engine speed and load while being skip fired. Because skip fire happens at low load it's likely that the extended time the system gets to fill the prechamber offsets this deactivated cylinder issue, but at the same time the operating cylinders are running richer and having the deactivated cylinders prechambers rich enough to fire may make the activated prechambers too rich causing misfires or combustion instability.
With these issues in mind, prechambers that are fed fuel in deactivated cylinders are likely to generate more NOx or HC or both. The generation of Non-Methane HC emissions is especially problematic as after the spark plug initiates combustion in the prechamber some unburned natural gas is pushed out of the prechamber into the main chamber before it is burned inside the prechamber.
The sixth portion of this background is most closely related to continuous water injection for ECI converted engines. Water injection has been used in engines to reduce engine knock at higher power levels as far back as World War 2. It was commonly used to allow aircraft engines to generate extra power during takeoff and other possible events that needed the most power possible.
It has also seen some use in racing applications, typically in sprint type racing where the time duration of full power and water injection use is limited, thus avoiding a bulky and heavy water storage system.
There are several issues that make water injection not worth the effort of implementation in most mobile applications; one is the volume and weight of the consumable water and second is the need to refill the container that would store it. Once these issues are overcome, then there is the environmental issue of keeping the stored onboard water from freezing when the vehicle is not in operation.
Another issue is the challenge of corrosion to the hardware that would be used to inject it, especially if the injector is designed to open and close rapidly for each cylinders combustion cycle.
Finally is the corrosion issue as related to any other parts. If after shut down an injector would leak water into the engine cylinder during engine storage, that cylinder will have internal corrosion and suffer significant maintenance issues.
Several Papers have indicated that direct injection of water into the engine cylinder has several advantages in addition to reducing engine knocking SAE paper 2009-01-1925 Effect of In Cylinder Water Injection Strategies on Performance and Emissions of a Hydrogen Fueled Direct Injection Engine is one good example. In this paper it is indicated that water injection both lowered NOx emissions and increased the indicated thermal efficiency when the water injection happened during the compression stroke. This effect was much less when the water was injected during the intake stroke on the four stroke engine tested.
When converting a diesel engine over to operate on natural gas, the compression ratio is typically reduced. If it wasn't reduced the engine may be limited to only generation of 60% of its original diesel operation rated power. The addition of water injection could allow the retention of higher compression ratios.